@Ted S posted:
I understand exactly what David is saying about the "timing." If you get a new wheelset, please try to compare it with the worn one. See if you can discern whether the position of the gear teeth is the same relative to the driving rod bolt. In other words, is the bolt perfectly lined up with a tooth or valley? Is it off by the same number of degrees? Etc. I imagine that it will be difficult to measure precisely. However... if the timing of the front and rear axles are way off, the side rod will be trying to pull the wheel forward before the worm gear applies pressure to the next tooth, or vice-versa.
The unique beauty of this design is that you can replace a wheelset without pulling or pressing anything. The bottom of the chassis comes off and you can slide the new wheelset right in, in place of the old one.
Ted, that's a great summary of the situation. I will add a second post addressing that, shortly.
Now, here's a top and bottom pic of my 671 chassis. I do not see any way to change out a wheel set short of pulling wheels, and in the case of the rear/front wheels, a wheel and a worm gear. What am I missing?